1. Field of the Invention
This invention relates generally to microwave and millimeter wave substrate technology and, more particularly, to a method for forming microwave and millimeter wave frequency solder connections to a substrate.
2. Description of the Related Art
Stripline, microstrip, and coplanar waveguide technologies are some of the well-known ways to propagate microwave frequencies across, and through printed circuit boards (PCBs), integrated circuit (IC) packages, and microwave integrated circuit (MIC) substrate. As used herein, microwave frequencies refer to frequencies generally in the range between 300 megahertz (MHz) and 30 gigahertz (GHz), while millimeter wave frequencies are greater than 30 GHZ. Also, the term substrate, as used herein, is the signal distribution body of an IC package. Conventionally, alumina ceramics (Al2O3), or glasses with ceramic loading, are used as substrates when the signal speeds are especially high, or the signal loss critical. It is also known to communicate between alumina substrate or PCB layers using vias and interlevel signal traces. However, one serious problem in the use of microwave to millimeter wave frequencies on a substrate is the interfacing of these signals to and from the substrate.
It is known to use leaded packages to form low microwave frequency connections from an IC package to a PCB. The leads typically extend from the side of the package, but can be surface mounted to the PCB when the leads are shaped by bending to be coplanar with the bottom surface of the package. However, this form of interface provides poor millimeter wave performance due to lead finger inductance and electromagnetic radiation of the signal in the curved leads. A straight lead version has improved millimeter wave performance, but cannot be surface mounted.
It is known to efficiently propagate microwave and millimeter wave frequencies through coaxial cables or waveguides. However, the connection of these transmission mediums to a substrate is cumbersome and expensive in high-volume manufacturing.
IC die connections can be made to the substrate using wire bonds. However, microwave to millimeter wave frequency performance of wire bonds is limited by the wire bond inductance. When an IC die is to be bonded, the chip bottom surface is typically attached to the substrate top surface, and relatively long wire bonds must be formed laterally across the substrate and die surfaces, between the substrate bonding pads and the pads on the die top surface.
An IC flip-chip die is a well-known semiconductor component that is conducive to low-cost circuit fabrication. The flip-chip die is formed with the bonding pads on the top surface. Either high temperature (PbSn or AuSn) or low temperature (eutectic SnPb) solder balls can be attached to these pads. When the die is to be attached to a substrate, the die is flipped so that the top surface overlies the substrate top surface, and its bonding pads are aligned over corresponding bonding pads on the substrate top surface. Heat is applied and the solder balls melt, connecting the pads of the flip-chip die to the pads of the substrate.
Dielectric film solder masks are typically used to help define solder connections to the substrate or PCB. The solder mask can be formed to selectively expose bonding areas where connections are made, or solder is applied. The remaining areas of solder mask act to contain the solder flow and prevent the accidental shorting of signal traces on the substrate or PCB with applied solder.
The solder mask openings on both sides of the solder connection control the solder reflow, and the surface tension of the molten solder causes the two bonding surfaces to stand a predetermined distance from each other.
To some extent, the problem of surface-mountable connections has been addressed by using flip-chip die attachment and grid ball array (BGA) connections to the PCB in the fabrication of cellular telephones. However, even cellular telephones rarely use signals higher than 2 gigahertz. Another huge commercial environment using flip-chip and BGA technology, the personal computer, barely uses low microwave frequency signals, and conventional connection techniques can be used to interface even the highest speed microprocessor IC to a package, and the package to a PCB. However, the advent of Internet and fiber optically related communications have brought forth the need for commercial microwave to millimeter wave frequency IC package and PCB connections.
At low microwave frequencies the dielectric film solder masks, discussed above, have negligible effect on signal propagation when they cover the signal traces on a PCB or substrate. However, at higher microwave and millimeter wave frequencies a dielectric film overlying a signal trace can seriously degrade signal propagation. The application of a solder mask over planar waveguides (having electrical lengths comparable to the radiation wavelength) causes a frequency-dependent dispersion in the propagating mode. Extremely thin-film dielectric overlying films can be used to mitigate the propagation problems, however, these films are not robust during the solder reflow process.
It would be advantageous if a low-cost commercial fabrication process could be developed for forming microwave and millimeter wave frequency interfaces to and from a substrate.
It would be advantageous if a process could be developed that would permit low-dispersion propagation at millimeter wave frequencies over signal traces that connect to a solder connection formed between a substrate and either a flip-chip die or a PCB.
It would be advantageous if conventional, well-known, selective deposition solder mask techniques could be adapted for use in forming microwave and millimeter wave frequency interfaces.
Accordingly, a microwave to millimeter wave frequency signal interface is provided. The signal interface comprises an alumina substrate top surface with bonding pads, and finite-extent solder dams surrounding the bonding pads. The solder dam is a thick-film dielectric that is selectively formed through a photo-imaging process. The solder dam is a either a fired glass ceramic or co-fired ceramic material having a firing temperature of greater than 800 degrees C.
Typically, the solder dam has a substantially circular-shaped interior diameter. The bonding surface area, inside the interior diameter is then defined by the solder dam. The solder dam has a wall width that minimally overlies any signal trace connected to the bonding pad. Thus, signal propagation along the trace is not degraded.
In some aspects of the invention, coplanar waveguide signal traces, with associated ground traces, are formed on the substrate, and connected to signal and associated ground bonding pads. The signal and associated ground bonding pads are separated by a center-to-center distance in the range between 0.003 to 0.05 inches. Finite-extent solder dams are formed around each ground and signal bonding pad.
Once the solder dams are formed, a solder joint can be formed having a solder joint dimension defined by the solder dam. The solder joint connects the substrate bonding pads to bonding pads of an overlying flip-chip integrated circuit die. In some aspects of the invention, the flip-chip IC die is a semiconductor device and the solder joints are formed by reflowing solder balls attached to the IC die bonding pads.
A method for forming a microwave to millimeter wave frequency interface is also provided. The method comprises: forming a substrate having a top surface with a bonding pad; forming a finite-extent solder dam surrounding the substrate top surface bonding pad; and, forming a solder joint from the substrate top surface bonding pad and an interfacing flip-chip IC die. Details of the interface formation process are included below.